(a) Field of the Invention
The present invention relates to membranes for making water treatment, and in particular, to a hollow fiber membrane module with enhanced cleaning efficiency and a method of manufacturing the same.
(b) Description of Related Art
Generally, the usage of membranes in treating purified water, sewage and waste water has been increasingly made since 1960s.
However, the conventional usage of membranes concerns higher degree treatment where the primarily treated water is re-treated using a separate membrane process to thereby obtain high quality treated water.
In order to obtain high quality treated water, an additional equipment should be provided to the existent facility, and hence, much cost should be consumed in installing, operating and maintaining it.
In this situation, a membrane bio reactor (MBR) has been significantly developed with the beginning of an experiment where hollow fiber membranes are directly dipped in a reactor to treat water, disclosed in an article with a title of “Direct Solid-Liquid Separation Using Hollow Fiber Membrane in an Activated Sludge Aeration Tank” by Kazuo Yamamoto et al. in 1989.
The MBR process involves a high concentration activated sludge of MLSS 8000-15000 ppm, compared to the conventional activated sludge process, and is operated to treat the sewage within a short period of time, thereby reducing the space required for the sewage treatment.
Furthermore, even with the presence of abnormalities such as shock loading, bulking and foaming which are mainly made with the activated sludge process using microorganisms with the poor precipitation of the activated sludge, treated water can be obtained in a stable manner.
However, the most serious problem with the MBR process is the phenomenon of membrane fouling. As the driving force of the membrane is largely due to the pressure difference, the activated sludge and other membrane contaminating materials within the reactor are accumulated at the membranes to thereby reduce the effective membrane surface area and decrease the amount of treated water.
In order to reduce the membrane fouling or contaminating phenomenon, the membrane materials have been studied by many researchers, and the techniques of physically inducing cross-flow on the surface of the membrane have been investigated.
The studies about the membrane materials have been made in the direction of reforming the surface of the membrane using surfactant or plasma. The physically-oriented process is conducted to exert a physical effect based on air, particularly, a cross flow on the membrane surface, and to sweep away the sludge cake accumulated on the membrane surface, or prevent the accumulation of the sludge cake.
In order to minimize the membrane contamination, various attempts have been made in the following ways.
In the case of the module of the Mitsubishi rayon company or the Zenon company, the treated water collectors are placed at both sides of the hollow fiber membranes, and the water treated through the membranes is collected at the both-sided collectors, followed by transferring it to a treatment water bath using a pump.
In case the treated water collectors are placed at both ends of the hollow fiber membranes, the both ends of the hollow fiber membranes are bound while limiting the movement of the hollow fiber membranes due to the air. Consequently, it is limited to obtain the desired membrane contamination reduction effect using the cross flow due to the air or the physical vibration due to the air.
Furthermore, as the two types of modules both involve two locations initially influenced by the pressure reduction (where the highest pressure difference is made), the accumulation of contaminants at those locations cannot be prevented.
With the module of the Zenon company, the air fed from the bottom goes up while elevating the activated sludge. The elevated activated sludge does not completely pass through the collector placed at the top of the module while being not fluently flown, and is stopped by the top collector so that it is again accumulated on the surface of the membranes due to the pressure difference at that location.
The membrane contamination (particularly due to the sludge cake) continuously propagates to the region where the pressure difference is relatively great, that is, the contamination degree is less, and as a result, the hollow fiber membranes are wholly contaminated. This phenomenon makes the cycle of the maintenance cleaning speedy, and as a result, the lifespan of the hollow fiber membranes is reduced.
Meanwhile, the article published by T. Ahmed et al. in 1992 with a title of “Use of Sealed-end Hollow Fibers for Bubbleless Membrane Aeration: experimental studies” discloses a module where the treated water collector is provided only at the one-sided front end of the hollow fiber membranes, although the usage thereof is different from that of the above module.
The Japanese Patent Application No. JP11128692 applied by the Toray company, the Japanese Patent Application No. JP10202270 applied by the Kuraray, the Korean Patent Publication No. 2001-112874 or the Korean Patent Publication No. 2002-39383 discloses a module similar to the above module.
However, the above structure also involves predictable problems. For instance, with the hollow fiber membrane module of the Toray company, only the bottom end of the hollow fiber membranes is fixed to the collector, and the hollow fiber membrane within the module is liable to be fallen. In operation, different air flows are abnormally made so that the hollow fiber membranes are tangled with each other. The physical stress is concentrated on the tangled portions of the hollow fiber membranes so that the hollow fiber membranes are cut at the tangled portions thereof, or at the interface thereof with the fixative.
The falling of the membranes within the module is not nearly made in the settling tank, but may be easily made when the hollow fiber membrane module is picked up from the aeration tank.
Furthermore, when the hollow fiber membranes within the module are tangled and broken, the balance of the diffuser is not maintained in a stable manner, and the water pressure applied to the diffuser is differentiated. The amount of air is small where the water pressure is high, and large where the water pressure is low so that the fluid flow from the latter to the former is made, and the hollow fiber membranes are directed along the flow, and tangled with each other.
Furthermore, when the hollow fiber membrane module is inclined in any one side, the locations of the module contacting the air flow are differentiated so that the fluid flow with a predetermined pattern is not induced. With the different fluid flows, a reverse flow is made within the module so that the hollow fiber membranes are tangled with each other. The tangled portions of the hollow fiber membranes continuously receive the physical stress of the fluid flows so that they are cut, or the whole stress is applied to the interface between the fixative and the hollow fiber membranes so that the hollow fiber membranes are cut at that interface.
The problematic falling of the hollow fiber membranes is easily made as the size of the hollow fiber membrane module becomes enlarged. As the conventional modules are not enlarged, many modules should be used to realize high capacity. Accordingly, the economical burden becomes increased to make, operate and maintain the relevant facilities.
Meanwhile, in order to manufacture a hollow fiber membrane module, the respective hollow fiber membranes should be made as a unit, and for this purpose, the hollow fiber membranes should be fixed to each other by interposing a thermoplastic or thermosetting material between the outer circumferences thereof.
It is most important to prevent the fixative from intruding the inner bore of the hollow fiber membrane and clogging it, and to prevent the generation of minute pores between the hollow fiber membrane and the fixative, through which the contaminants are input.
In this connection, the one-sided inner bore of the hollow fiber membrane is first fired or blocked by using another material, and after the hollow fiber membrane is completely fixed using a fixative, cut by a predetermined height, followed by recovering the inner bore thereof. In order to cut the bottom end portion of the hollow fiber membrane, a band saw, a sharp knife, a hydraulic cutter or a laser may be used.
With the conventional structure, the secondary wastes of the fixative and the hollow fiber membrane pieces after the cutting cannot be recycled, and induce economical loss. Furthermore, with the usage of a band saw, the pieces of the fixative broken due to the band saw clog the inner bore of the hollow fiber membrane, thereby inducing the pressure loss. In case a knife or a hydraulic cutter is used while requiring high pressure or power, the fixative is liable to be detached from the mold so that the contaminants are flown into the treated water.
The above problems are more frequently made as the module becomes enlarged. The laser technique is advantageous in enlarging the module, but the facility cost related thereto is high.
Meanwhile, the most important problem with the conventional techniques concerns the packing density of the hollow fiber membrane module.
The packing density of the module depends upon how many hollow fiber membranes can be packed within the module at a unit area. With the conventional submerged type hollow fiber membrane module, the packing density thereof is at best 10-25%, and hence, many modules should be used to treat the waste water, incurring economical loss.
The fixative for fixing the hollow fiber membranes to the mold has a relatively high viscosity, and as it is solidified, the viscosity thereof becomes increased while making it difficult for the fixative to intrude the gap between the hollow fiber membranes. Consequently, many hollow fiber membranes cannot be incorporated within a mold.
Furthermore, in case the module is formed without arbitrarily spacing the hollow fiber membranes from each other, even if the packing density becomes lowered, some of the hollow fiber membranes may be densely grouped while making it difficult for the fixative to intrude between the hollow fiber membranes.
In order to solve the problematic intrusion of the fixative, with the Zenon company, as disclosed in US006294039B1 or US006042677A, hollow fiber membranes are arranged and attached on a tape such that the distances between the hollow fiber membrane neighbors are constantly maintained, or the same effect is made using an adhesive.
However, with the conventional structure for making the fixative easily intrude the gap between the hollow fiber membranes, as the hollow fiber membranes are arbitrarily spaced apart from each other, it cannot be expected to achieve high packing density.
Furthermore, US Patent No. 20010037967A1 and US Patent No. 20020153299A1 disclose a method of using two different materials in forming a hollow fiber membrane module. In the former case where a liquid-phased filler is used as a temporary fixative, the liquid filler rides along the hollow fiber membranes due to the capillary phenomenon, and fills the space for a permanent fixative so that the permanent fixative cannot intrude between the hollow fiber membranes.
In the latter case where the temporary fixative is based on solid particulate powder, the powdered fixative should thoroughly intrude the inner bore of the hollow fiber membranes as well as the gap between the hollow fiber membranes.
However, with the conventional technique, it is difficult to fill the solid-phased temporary fixative between the hollow fiber membranes by a predetermined height. In case the temporary fixative has a relatively large size, the space therefor becomes widened so that the permanent fixative may intrude and clog the inner bore of the hollow fiber membranes. In case a water-insoluble temporary fixative is used, it is difficult to completely recover the temporary fixative. In such a case, the temporary fixative is flown to the treated water, thereby deteriorating the quality thereof.